WO1997016581A2

WO1997016581A2 - Low-nitrate, manganese-free zinc phosphatization

Info

Publication number: WO1997016581A2
Application number: PCT/EP1996/004541
Authority: WO
Inventors: Jan-Willem Brouwer; Jürgen Geke; Karl-Heinz Gottwald; Peter Kuhm
Original assignee: Henkel Kommanditgesellschaft Auf Aktien
Priority date: 1995-10-27
Filing date: 1996-10-18
Publication date: 1997-05-09
Also published as: WO1997016581A3; DE19540085A1; AU7296096A

Abstract

A process is disclosed for phosphatizing steel, galvanized or alloy galvanized steel and/or aluminium or its alloys by treatment with a zinc phosphatizing solution applied in a dip, spray or spray/dip process. The process is characterized in that the zinc phosphatization solution has a maximum nitrate ion content of 0.5 g/l, is free of manganese, nickel and cobalt ions, contains 0.3-2 g/l zinc ions, 5-40 g/l phosphate ions and one or more accelerators.

Description

Nitrate, nangane-free zinc phosphating

The invention relates to a process for phosphating metal surfaces with aqueous, acidic phosphating solutions which contain zinc and phosphate ions and accelerators in free or bound form, and to their use as pretreatment of the metal surfaces for subsequent painting, in particular electrocoating. The method can be used for the treatment of surfaces made of steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel.

The phosphating of metals pursues the goal of producing firmly adhered metal phosphate layers on the metal surface, which in themselves improve corrosion resistance and, in conjunction with lacquers and other organic coatings, to a substantial increase in adhesion and resistance to migration under corrosion contribute. Such phosphating processes have long been known in the prior art. The low-zinc phosphating processes, in which the phosphating solutions are comparatively low in zinc ion contents of z. B. 0.5 to Have 2 g / 1. An important parameter in these low-zinc phosphating baths is the weight ratio of phosphate ions to zinc ions, which is usually in the range> 12 and can take values up to 30.

It has been shown that by using other polyvalent cations than zinc in the phosphating baths, phosphate layers with significantly improved corrosion protection and paint adhesion properties can be formed. For example, find low-zinc processes with the addition of z. B. 0.5 to 1.5 g / 1 manganese ions and z. B. 0.3 to 2.0 g / 1 of nickel ions as a so-called trication process for preparing metal surfaces for painting, for example for the cathodic electrocoating of car bodies, widely used.

DE-A-39 20 296 describes a phosphating process which dispenses with nickel and uses magnesium ions in addition to zinc and manganese ions. The phosphating baths described here contain, in addition to 0.2 to 10 g / 1 nitrate ions, further oxidizing agents which act as accelerators, selected from nitrite, chlorate or an organic oxidizing agent. Concerns about the accelerators nitrite and nitrate are being raised due to the possible formation of nitrous gases.

The phosphating process disclosed in WO 86/04931 works without nitrates. Here the accelerator system is based on a combination of bromate and m-nitrobenzenesulfonate. Only zinc is given as the essential polyvalent cation, and nickel, manganese or cobalt is given as further optional cations. In addition to zinc, the phosphating solutions preferably contain at least 2 of these optional metals. DE-A-40 13 483 discloses phosphating processes with which good corrosion protection properties can be achieved as with the trication processes. These processes do without nickel and instead use copper in low concentrations, 0.001 to 0.03 g / 1. Oxygen and / or other oxidizing agents having the same effect are used to oxidize the divalent iron formed in the pickling reaction of steel surfaces to the trivalent stage. Nitrite, chlorate, bromate, peroxy compounds and organic nitro compounds, such as nitrobenzenesulfonate, are specified as such. The German patent application DE 42 10513 modifies this process by adding hydroxylamine, its salts or complexes in an amount of 0.5 to 5 g / 1 hydroxylamine as a modifying agent for the morphology of the phosphate crystals formed.

The use of hydroxylamine and / or its compounds to influence the shape of the phosphate crystals is known from a number of published publications. EP-A-315059 specifies as a special effect of the use of hydroxylamine in phosphating baths the fact that on steel the phosphate crystals are still formed in a desired columnar or knot-like form when the zinc concentration in the phosphating bath is the same for low-zinc Ver ¬ driving exceeds the usual range. This makes it possible to operate the phosphating baths with zinc concentrations of up to 2 g / 1 and with weight ratios of phosphate to zinc down to 3.7. No further statements are made about advantageous cation combinations of these phosphating baths, but nickel is used in all patent examples. Nitrates and nitric acid are also used in the patent examples, even if the description of the presence of nitrate in large amounts is not advised in the description. The required hydroxylamine concentration is given as 0.5 to 50 g / 1, preferably 1 to 10 g / 1. Trication-phosphating processes, in which the phosphating solutions contain zinc, nickel and manganese ions, were developed especially for the phosphating of galvanized steel. The incorporation of nickel and manganese in the phosphate layers compensated for disadvantages with regard to paint adhesion and corrosion protection, which resulted from the fact that only hopeit crystals, but no phosphophyllite crystals, were formed on galvanized substrates. It is known from the phosphating of non-galvanized steel that layers rich in phosphorphyllite have advantages over layers rich in hopite in terms of paint adhesion and corrosion protection. Since galvanized steel is widely used in automobile construction, trication-phosphating processes are currently widespread for this area of application. The presence of nickel and manganese in almost the same concentrations as zinc leads to an increased formation of sludge containing heavy metals in the wastewater treatment of rinsing water, which must be disposed of as special waste. When refinishing nickel-containing phosphate layers, nickel-containing dusts can also form, which are classified as carcinogenic.

For this reason, efforts have been made for a number of years to develop phosphating processes in which the particularly critically rated nickel can be dispensed with without sacrificing performance. Among other things, consideration was given to replacing nickel with cobalt. However, at least from the point of view of wastewater technology and toxicology, cobalt is not an alternative to nickel. In the above, reference has already been made to some documents in which, for example, magnesium or copper, the latter in very low concentrations, is used instead of nickel. The object of the present invention is now to provide phosphating processes in which not only the toxicologically questionable metals nickel and cobalt are dispensed with, but also manganese. As a result, the heavy metal content of the sludge obtained in the treatment of the rinse water can be significantly reduced. Of course, the new phosphating process demands that phosphate layers can be produced as a basis for subsequent painting, the properties of which do not lag behind those of manganese-containing processes and especially trication processes.

This object is achieved by a method for phosphating steel, galvanized or alloy-galvanized steel and / or aluminum or its alloys by treatment with a zinc phosphating solution in the immersion, spray or spray immersion process, characterized in that the zinc phosphating solution paints a maximum Has nitrate ion content of 0.5 g / 1 and is free of manganese, nickel and cobalt ions and that they

0.3 to 2 g / 1 zinc ions, 5 to 40 g / 1 phosphate ions

and one or more of the following accelerators:

0.1 to 10 g / 1 hydroxylamine in free, ionic or complex-bound form,

0.3 to 5 g / 1 chlorate ions,

0.05 to 2 g / 1 m-nitrobenzenesulfonate ions,

0.05 to 2 g / 1 m nitrobenzoate ions

0.05 to 2 g / 1 p-nitrophenol 0.005 to 0.15 g / 1 hydrogen peroxide in free or bound

Form, 0.01 to 10 g / 1 of a reducing sugar.

The fact that the phosphating baths are free from manganese, nickel and cobalt ions means that these ions are not deliberately added to the phosphating baths. When phosphating metal surfaces which contain these metals as alloy components, it cannot be ruled out that small amounts of the corresponding cations may get into the phosphating baths as a result of the pickling attack. This can be the case in particular when steel coated with a zinc-nickel alloy is phosphated. In practice, however, the expectation is placed on the phosphating baths that they contain less than 0.05 g / 1 of the cations mentioned.

In addition to zinc ions, phosphating baths generally contain sodium, potassium and / or ammonium ions to adjust the free acid. The term free acid is familiar to those skilled in the phosphating field. The method of determining free acid and total acid selected in this document is given in the example section. In the phosphating process according to the invention, preference is given to using phosphating solutions which contain further mono- or divalent metal ions which, owing to their chemical properties and / or their lower use concentrations, are less problematic than nickel and manganese and which, according to experience, are favorable affect the paint adhesion and the corrosion protection of the phosphate layers produced with it. Accordingly, the process according to the invention is preferably characterized in that the phosphating solution additionally contains one or more of the following cations:

0.2 to 2.5 g / 1 magnesium (II), 0.2 to 2.5 g / 1 calcium (II),

0.01 to 0.5 g / 1 iron (II),

0.001 to 0.03 g / 1 copper (I) and / or copper (II),

0.2 to 1.5 g / 1 lithium (I),

0.02 to 0.8 g / 1 tungsten (VI).

The weight ratio of phosphate ions to zinc ions in the phosphate baths can vary within wide limits, provided it is in the range between 3.7 and 30. A weight ratio between 10 and 20 is particularly preferred. The skilled worker is familiar with the free acid and total acid contents as further parameters for controlling phosphating baths. The method of determining these parameters used in this document is given in the example section. Values of the free acid between about 0.3 and about 1.5 points and the total acid between about 15 and about 35 points are in the technically customary range and are suitable for the purposes of this invention.

When phosphating surfaces containing zinc, it has proven to be advantageous to limit the nitrate content of the phosphating bath to a maximum of 0.5 g / l. This suppresses the problem of so-called speck formation and improves corrosion protection. Phosphating baths containing less than 0.05 g / l and in particular no nitrate are particularly preferred.

In phosphating baths which are said to be suitable for different substrates, it has become customary to add free and / or complex-bound fluoride in amounts of up to 2.5 g / 1 total fluoride, of which up to 800 mg / l free fluoride. The presence of such amounts of fluoride is also advantageous for the phosphating baths according to the invention. In the absence of fluoride, the aluminum content of the bath should not exceed 3 mg / l. In the presence of fluoride tolerated higher AI contents due to the formation of the complex, provided the concentration of the non-complexed AI does not exceed 3 mg / l.

Suitable accelerators are the substances listed above and known in the prior art. On the one hand, these accelerate the layer formation, since they have a “depolarizing” effect by oxidizing the elemental hydrogen which forms on the metal surface during the pickling attack to water. However, certain accelerators such as hydroxylamine can also influence the shape of the phosphate crystals that are formed. Oxidizing accelerators also lead to the fact that iron (II) ions formed on steel surfaces during the pickling reaction are oxidized to the trivalent stage, so that they precipitate out as iron (III) phosphate.

In addition to the accelerators listed above, nitrite could also be used within the scope of the invention. However, this is less preferred since the present invention is intended to provide a phosphating process which has particularly few toxicological and disposal problems. From the point of view of occupational safety, however, nitrite is to be assessed as particularly critical, since nitrous gases which are harmful to health can be generated when it is used. Furthermore, when nitrite is used, nitrate could be formed by its oxidation, so that the low nitrate contents according to the invention would be exceeded.

The accelerators listed above can be used individually, but in chemical compatibility they can also be used in combination. For example, a preferred embodiment of the invention is that a combination of chlorate ions and hydrogen peroxide is used in the phosphating in the immersion process. The hydrogen peroxide can 7/16581 PO7EP96 / 04541

such d. H. in free form or in bound form, for example as ionic peroxide or in the form of peroxo compounds such as peroxosulphuric acid, Caro's acid or also peroxophosphoric acid. Sodium perborate can be used as a further carrier for hydrogen peroxide in bound form. In this embodiment, the concentration of chlorate can be, for example, in the range from 2 to 4 g / l, the concentration of hydrogen peroxide in the range from 10 to 50 ppm.

The use of reducing sugars as accelerators is known from US-A-5 378 292. In the context of the present invention, they can be used in amounts between about 0.01 and about 10 g / 1, preferably in amounts between about 0.5 and about 1.5 g / 1. Examples of such sugars are galactose, mannose and in particular glucose (dextrose).

Another preferred embodiment of the invention consists in using hydroxylamine as accelerator. Hydroxylamine can be used as a free base, as a hydroxylamine complex or in the form of hydroxylammonium salts. If free hydroxylamine is added to the phosphating bath or a phosphating bath concentrate, it will largely be present as a hydroxylammonium cation due to the acidic nature of these solutions. When used as a hydroxylammonium salt, the sulfates and the phosphates are particularly suitable. In the case of the phosphates, the acid salts are preferred because of their better solubility. Hydroxylamine or its compounds are added to the phosphating bath in amounts such that the calculated concentration of the free hydroxylamine is between 0.1 and 10 g / 1, preferably between 0.3 and 5 g / 1. It is preferred that the phosphating baths contain hydroxylamine as the sole accelerator, at most together with a maximum of 0.5 g / l of nitrate. Accordingly, in a preferred Embodiment phosphating baths are used which do not contain any of the other known accelerators such as nitrite, oxo anions of halogens, peroxides or nitrobenzenesulfonate.

In practice it has been shown that the accelerator hydroxylamine can be slowly inactivated even if no metal parts to be phosphated are introduced into the phosphating bath. It has surprisingly been found that the inactivation of the hydroxylamine can be significantly slowed down if one or more aliphatic hydroxycarboxylic acids having 3 to 6 carbon atoms in a total amount of 0.5 to 1.5 g / l are added to the phosphating bath . The hydroxycarboxylic acids are preferably selected from lactic acid, gluconic acid, tartronic acid, malic acid, tartaric acid and citric acid, citric acid being particularly preferred.

The zinc content of the phosphating bath will be set according to EP-A-315059 to values between 0.45 and 1.1 g / 1. As a result of the pickling removal in the phosphating of zinc-containing surfaces, however, it is possible that the current zinc content of the working bath is above a value of 1.1 g / l. Zinc levels up to 2 g / l are harmless in the context of the present invention. Depending on the plant technology, zinc contents of up to 2 g / l can also reduce the risk of rust formation during phosphating. The form in which the cations are introduced into the phosphating baths is in principle irrelevant. It is particularly useful to use oxides and / or carbonates as the cation source.

When the phosphating process is used on steel surfaces, iron dissolves in the form of iron (II) ions. Since the phosphating baths according to the invention preferably do not contain any substances contain that have an oxidizing effect on iron (II), the divalent iron only changes into the trivalent state as a result of air oxidation, so that it can precipitate out as iron (III) phosphate. Therefore, iron (II) contents can be built up in the phosphate baths according to the invention which are clearly above the contents which contain baths containing oxidizing agents. In this sense, iron (II) concentrations of up to 50 ppm are normal, although values of up to 500 ppm can also occur briefly in the production process. Such iron (II) concentrations are not detrimental to the phosphating process according to the invention. When prepared in hard water, the phosphating baths can further contain the hardness-forming cations Mg (II) and Ca (II) in a total concentration of up to 7 mmol / l. Mg (II) or Ca (II) can also be added to the phosphate bath in amounts of up to 2.5 g / l.

In a further preferred embodiment, a phosphate solution is used which is the only one containing cations with an actual or potential oxidation state of> 1 zinc and copper ions. Zinc and copper ions will generally be present in oxidation level 2 in the phosphate solution. However, it is conceivable that accelerators with a principally reducing action, such as, for example, hydroxylamine, can at least partially reduce the copper ions to the monovalent stage. In addition to zinc and copper ions, this preferred phosphating solution can also contain sodium, potassium and / or ammonium ions, which, as usual, can be used in the form of their basic compounds to adjust the free acidity of the phosphating solution. It is particularly preferred that the phosphating solution contains 3 to 20 mg / l copper ions when used in the dipping process and 1 to 10 mg / l copper ions when used in the spraying process. The temperature of the phosphating solution is preferably in the range between about 40 and about 60 ° C. It has emerged as a positive side effect of the phosphating process according to the invention that hydroxylamine concentrations above approximately 1.5 g / l significantly reduce the risk of rust formation at insufficiently flooded areas of the components to be phosphated.

The method according to the invention is suitable for phosphating surfaces made of steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel. The materials mentioned can also be present side by side, as is becoming increasingly common in automobile construction. Parts of the bodywork can also consist of material that has already been pretreated, such as is produced using the Bonazink ^R process. In this case, the base material is first chromated or phosphated and then coated with an organic resin. The phosphating process according to the invention then leads to phosphating on damaged areas of this pretreatment layer or on untreated rear sides.

The method can be used in particular in automotive engineering, where treatment times between 1 and 8 minutes are common. It is intended in particular for the treatment of the metal surfaces mentioned before painting, in particular before cathodic electrocoating, as is customary in automobile construction. The phosphating process is to be seen as a sub-step of the technically usual pretreatment chain. In this chain, the steps of cleaning / degreasing, rinsing and activating are usually preceded by the phosphating, the activation usually being carried out using activating agents containing titanium phosphate. The phosphating according to the invention can be followed, if appropriate after an intermediate rinse, by a passivating aftertreatment. For such a passivating aftertreatment, treatment baths containing chromic acid are widespread. For reasons of occupational and environmental protection and for reasons of disposal, however, there is a tendency to replace these chromium-containing passivation baths with chromium-free treatment baths. Purely inorganic baths, in particular based on zirconium compounds, or also organic baths, for example based on poly (vinylphenols), are known for this. An intermediate rinse with demineralized water is generally carried out between this post-passivation and the subsequent electro-dip coating.

embodiments

The phosphating processes according to the invention and comparative processes were checked on electrolytically galvanized steel sheets, as are used in automobile construction. The following process step, customary in body production, was carried out as an immersion process:

1.Clean with an alkaline cleaner (Ridoline ^R 1559, Henkel KGaA), mix 2% in city water, 55 ° C, 4 minutes.

2. Rinse with city water, room temperature, 1 minute.

3. Activation with an activating agent containing titanium phosphate (Fixodine ^R C9112, Henkel KGaA), mixture 0.1 H in fully salted water, room temperature, 1 minute.

4. Phosphating with phosphating baths according to Table 1. Temperature 55 ° C. In addition to the cations listed in Table 1, the phosphating baths only contained sodium ions to adjust the free acid. The baths contained no nitrite or nitrate.

The free acid score is understood to mean the consumption in ml of 0.1 normal sodium hydroxide solution in order to titrate 10 ml of bath solution up to a pH of 3.6. Similarly, the total acid score indicates consumption in ml up to a pH of 8.5.

5. Rinse with city water, room temperature, 1 minute. 6. Post-passivation with a chrome-free post-passivation agent based on complex zirconium fluoride (Deoxylyte ^R 54 NC, Henkel KGaA) 0.25% in deionized water, pH 4.0, temperature 40 ° C, 1 minute.

7. Rinse with deionized water.

8. Blow dry with compressed air

The mass per unit area ("layer weight") was determined by dissolving in 5% chromic acid solution in accordance with DIN 50942.

The phosphated test sheets were coated with a cathodic dip coating from BASF (FT 85-7042). The corrosion protection effect was tested in an alternating climate test according to VDA 621-415 over 10 rounds. As a result, the paint infiltration at the scratch (half scratch width) is shown in Table 1. Table 1 also contains the results of a stone chip test according to the VW standard as "K values".

Table 1 Phosphating baths and test results

Comp. 1 Comp. 2 Ex. 1 Ex. 2

Zn (g / 1) 1.1 1.1 1.1 1.1

Mn (g / 1) 0.9 0.9 - -

Cu (g / 1) - 0.01 0.01 -

PO ₄ ³ - (g / D 20 15 15 15

CIO3- (g / 1) 3 - 3 -

H ₂ 0 ₂ (g / D 0.03 - 0.03 -

Hydroxylammonium - 2.0 - 2.0 umulfate (g / 1)

SiF ₆ 2 "(g / 1) 1.0 1.0 1.0 1.0

Free acid (points) 1.1 1.1 1.1 1.1

Total acidity (points) 28 23 22 23

Dive time (minutes) 3 3 3 3

Layer weight (g / m2) 3.5 3.5 3.5 3.9

Lacquer base wall 2.4-3.1 n.b. *) 2.6-3.1 n.b. *) tion (mm)

Rockfall code 8/8 n / a *) 8/8 n / a *) worth (K)

nb *) = not determined Table 1 - continued

Ex. 3

Zn (g / 1) 1.1

Mn (g / 1) -

Cu (g / 1) 0.01

P0 ₄ ^{3 "} (g / D 15

CIO3- (g / 1) -

H ₂ 0 ₂ (g / 1) -

Hydroxylammonium 2.0 um sulfate (g / 1)

SiF ₆ 2- _{(g / 1)} 1.0

Free acidity (points) 1.1

Total acidity (points) 23

Diving time (minutes) 3

Layer weight (g / m2) 2.9

Lacquer base wall *) (mm)

Rockfall characteristic n.b. *) value (K)

n.b. *) = not determined

Claims

claims

1. A process for phosphating steel, galvanized or alloy galvanized steel and / or aluminum or its alloys by treatment with a zinc phosphating solution in the immersion, spray or spray immersion process, characterized in that the zinc phosphating solution has a maximum nitrate ion content of 0. 5 g / 1 and is free of manganese, nickel and cobalt ions and that they

0.3 to 2 g / 1 zinc ions, 5 to 40 g / 1 phosphate ions

and one or more of the following accelerators:

0.1 to 10 g / 1 hydroxylamine in free, ionic or complex-bound form,

0.3 to 5 g / 1 chlorate ions,

0.05 to 2 g / 1 m-nitrobenzenesulfonate ions,

0.05 to 2 g / 1 m nitrobenzoate ions

0.05 to 2 g / 1 p-nitrophenol

0.005 to 0.15 g / 1 hydrogen peroxide in free or bound form

0.01 to 10 g / 1 of a reducing sugar.

2. The method according to claim 1, characterized in that the phosphating solution additionally contains one or more of the following cations:

0.2 to 2.5 g / 1 magnesium (II), 0.2 to 2.5 g / 1 calcium (II), 0.01 to 0.5 g / 1 iron (II),

0.001 to 0.03 g / 1 copper (I) and / or copper (II),

0.2 to 1.5 g / 1 lithium (I),

0.02 to 0.8 g / 1 tungsten (VI).

3. The method according to one or both of claims 1 and 2, characterized in that the phosphating solution contains no more than 0.05g / l nitrate ions.

4. The method according to one or more of claims 1 to 3, characterized in that the phosphating solution additionally fluoride in free and / or complex-bound form in amounts up to 2.5 g / 1 total fluoride, of which up to 0.8 g / 1 contains free fluoride.

5. The method according to one or more of claims 1 to 4, characterized in that the phosphating solution when used in the immersion process as an accelerator contains a combination of chlorate ions and hydrogen peroxide in free or bound form.

6. The method according to one or more of claims 1 to 4, da¬ characterized in that the phosphating solution as the only accelerator contains hydroxylamine in a concentration in the range 0.3 to 5 g / 1.

7. The method according to claim 6, characterized in that the phosphating solution additionally contains one or more aliphatic hydroxycarboxylic acids with 3 to 6 carbon atoms in a total amount of 0.5 to 1.5 g / 1.

8. The method according to claim 7, characterized in that the hydroxycarboxylic acids are selected from lactic acid, gluconic acid, tartronic acid, malic acid, tartaric acid and citric acid

9. The method according to one or more of claims 2 to 8, characterized in that the phosphating solution contains as the only cations with an actual or potential Oxidations¬ stage of greater than 1 zinc and copper ions.

10. The method according to claim 9, characterized in that the phosphating solution contains 3 to 20 mg / l copper ions when used in the dipping process, and 1 to 10 mg / l copper ions when used in the spraying process.

11. Use of the method according to one or more of claims 1 to 9 for pretreating the metal surfaces before painting.